It is known that current avionic architectures generally comprise, in a conventional manner, a flight management system (FMS) which offers the crew the possibility of defining before the flight the route to be followed in order to take the passengers to their destination, and of maintaining or changing this route during the flight.
Generally, this flight management system also has the function of calculating, on the basis of the route defined in this manner by the crew, a trajectory (in particular a so-called “5D” trajectory which takes into account the following parameters: altitude, speed, quantity of fuel, time and wind encountered), on which the guiding system of the aircraft will have to base the position of the aircraft.
This calculation of the trajectory is carried out in an iterative manner since the lateral and vertical portions are strongly linked and several calculation loops are required before converging towards a precise solution.
Two examples of these interactions between the lateral and vertical portions of the trajectory are given below by way of illustration:                firstly, in order to determine the turning radii at each transition between two segments of the flight plan, the system has to know the speed at which the aircraft will fly the transition and vice versa. Furthermore, in order to calculate all the predictions along the trajectory and therefore in particular the speed profile, the system needs to know precisely the total length of the trajectory (including the turning radii of the transitions). Several iterations with increasingly fine hypotheses are therefore required in order to calculate the transitions, then the total length of the trajectory and to loop regarding the evaluation of the speed profile until the desired level of precision is reached;        secondly, the flight plan may include some types of segments (or legs) which are referred to as “floating”, such as, for example, legs which terminate at a given altitude and not at a fixed point, for which the geometry of the lateral trajectory is dependent on the performance of the aircraft. The lateral trajectory, and with it the length of the trajectory, change in accordance with the point at which the aircraft reaches the altitude of the leg. In this example, several successive calculations are required in order to refine the results and to converge towards the final trajectory.        
These two examples show that the calculations and in particular the calculations of performance levels implemented in a flight management system are increasingly important and make use of various and numerous calculation means (or integrators) which are combined.
As a result of an increasing number of functions and an increased demand in terms of calculations, in particular in terms of performance calculations, which are dependent on each other, the flight management system (which comprises in particular means for defining a lateral trajectory and means for calculating predictions along this lateral trajectory, implementing performance functions) becomes increasingly complex with in particular disadvantages in terms of change and redundancy of development with other systems.